1. Field of the Invention
Our invention relates generally to low temperature mechanical refrigeration systems and, more specifically, systems which quick freeze and/or store food products, such as novelty ice cream products. Such systems typically include a hardening tunnel through which the products pass on a conveyor system. As such products pass through the hardening tunnel, low temperature air is passed over the product to blast freeze it. The temperature of the air passing over the product must be maintained constant to maintain uniformity in the appearance and condition of the product. Any foreign substance added to the system during operation must not contaminate the food products. Finally, bacterial contamination of the product must be prevented. Our invention provides a novel method that does not affect the normal continuous operation of the refrigeration system, thereby obviating the necessity for frequent defrosting. It uses intermittent, frequent spraying of the external surface of the evaporator coil of such a system with a food-grade aqueous propylene glycol solution to provide essentially frost-free continuous operation.
Because moisture is present in such refrigeration systems, over time, evaporator coils become coated with a build-up of frost causing reduced unit capacity and decreased air flow. Frost build-up also decreases product load capability per ton of compressor capacity. In a typical system, the evaporator coils must defrosted frequently. Our invention also provides an apparatus for defrosting that maintains high air velocity throughout, thereby reducing the size of the equipment. The high air velocities also increase heat transfer coefficients in the evaporator coil area, thus reducing the evaporation surface area required for a given cooling capacity.
2. Description of the Prior Art
Known techniques for defrosting the evaporator coils have serious shortcomings. For example, one popular response to the build-up of frost on evaporator coils is to apply heat, in the form of hot gasses, to the coils to melt the frost. This approach is disclosed in U.S. Pat. No. 3,922,875 (Morris), which claims a reversible cycle refrigeration system. There, the refrigeration system is cycled alternatively through a cooling phase and a defrosting phase. During the cooling phase, liquid refrigerant is supplied to the evaporator coils for cooling. In the defrosting phase, the cold refrigerant supply is stopped and hot gaseous refrigerant from the high pressure discharge of the compressor is routed to the evaporator coil to melt the frost. A similar system was disclosed in U.S. Pat. No. 4,736,594 (Pao).
Such cyclic defrosting systems have several disadvantages. First, additional energy is required to heat the gaseous refrigerant during the defrosting phase. Second, additional refrigeration is then required to remove the heat transferred to the evaporator coil and the unit casing by the hot refrigerant gas used to defrost the evaporator coil. This decreases overall thermal efficiency. Third, in these systems no provision is made to remove any bacteria which may be introduced into the system during defrosting. Fourth, such units must be shut-down during the defrosting operation, resulting in loss of valuable production time. Finally, repeated heating and cooling of the evaporator coils can cause thermal fatigue, material stresses, and premature failure. Thus, the evaporator coils in such systems must be replaced frequently.
A second approach to the problem of frost build-up, a refrigeration system which uses sprayed glycol, or a similar solution, to defrost the evaporator coil is disclosed in U.S. Pat. No. 3,300,993 (Schlemmer). In that system, frost is removed from the evaporator coil by continuously spraying a propylene glycol or brine solution over the coil. The solution trickles down the coil and is collected in a sump. This system must operate at low air velocities to prevent carry-over of the solution. Therefore, it must be sized to accommodate the large air ducts necessary to provide the low air velocities required to prevent glycol entrainment. Such low air velocities inherently reduce the overall heat transfer film coefficient outside the evaporator tubes, thus necessitating increased surface area.
Similarly, U.S. Pat. No. 3,805,538 (Fritch et al.), discloses an apparatus and method for freezing food items in three stages The second stage subjects the items to a blast of refrigerated air. In this stage, the evaporator coil is sprayed with a propylene glycol solution which drips to the floor where it is collected. Any solution entrained in the air is removed by gravity as the air is subsequently decelerated from about 540 fpm. to about 260 fpm. when the diameter of the duct section suddenly increases. This sudden reduction and subsequent increase in air velocity increases the required fan horsepower. Like the system disclosed by Schlemmer, the air velocity must be relatively low throughout in order to limit carry-over of the glycol solution.
Both of the previously disclosed approaches to the problem of frost build-up have serious flaws. The cyclic defrost systems must be shut down periodically to allow hot refrigerant gas to defrost the evaporator coil. Previously-known glycol spraying systems cannot be operated at high air velocities, or at the least must include sudden decreases in air velocity which increase the power needed to operate the system.